The present study explored whether the N400 semantic priming effect is based on semantic similarity or lexical associations, or both. The event-related potentials showed N400 priming effects for both semantically similar and lexically associated word pairs in the 250-375 ms time-window. However, the effect lasted for a longer time for lexical associates, particularly in frontal and central electrode sites in the 375-500 ms time-window, suggesting that different types of processing may contribute to the N400 priming effects evoked by the two types of relationships.